In general, heat pipes, and the concepts which surround their use, are well recognized by the prior art. A heat pipe is a closed, constant mass heating system in which the system liquid coexists in equilibrium with its vapor during normal operating temperatures. It consists of an evaporator section, in which the system liquid is heated and vaporized; an adiabatic section, in which both vapor and condensed liquid flow with no heat being externally transferred; and a condensor section, in which the latent heat of vaporization is transferred to the external surroundings, and the condensed liquid flows via capillary action back towards the evaporator. The axial flow of the vapor and the capillary flow of the returning system liquid are both produced by pressure gradients, that are created by the interaction between naturally-occurring pressure differentials within the heat pipe. These pressure gradients eliminate the need for external pumping of the system liquid. In addition, the existence of liquid and vapor in equilibrium, under vacuum conditions, results in higher thermal efficiencies.
Heat pipes, as mentioned above rely for their operation upon the existence of the induced pressure gradients, which work to force vapor flow toward the condenser, and capillary liquid flow back toward the evaporator.
In order to increase the efficiency of heat pipes, various wicking structures have been developed to promote liquid transfer between the condensor and evaporator sections. They have included longitudially disposed parallel grooves and the random scoring of the internal pipe surface. In addition, the prior art also discloses the use of a wick structure which is fixidly attached to the internal pipe wall. The compositions and geometries of these wicks have included, a uniform fine wire mesh, and circumferentially disposed fine wire hoops of varied spacing.
All of the geometries, either integral to the internal pipe wall or integral to the affixed wick, are chiefly designed to promote liquid and vapor flow while maintaining high thermal efficiencies through the pipe wall to the ambient surroundings.
In general, the wick structures disclosed in the prior art provide grooves for condensate return to the evaporator, such that evaporator "dryout" is minimized. However, these internal structures fail to accomodate the different requirements and functions of each particular section of the heat pipe, and therefore they do not produce the optimum output available from structures of this type.
Examples of some of the aforementioned prior art devices may be seen by reference to U.S. Pat. Nos. 4,109,709; 4,116,266; 4,058,159; 4,274,479; and 4,186,796.
These prior art devices, while adequate for their intended purpose, suffer from the common deficiency, in that they do not fully realize the optimum inherent heat transfer potential available from a given heat pipe.
To date, no one has devised a wick structure for a heat pipe, which is simple to produce, and yet provides optimum heat transfer characteristics for the heat pipe in which it is utilized.